Re: [RESEND PATCH] KVM: x86/mmu: make apf token non-zero to fix bug

[Xinlong Lin]

Thank you and sorry for the noise.

On 2022/3/10 11:42, zhangliang (AG) wrote:
> No. Because '(++vcpu->arch.apf.id << 12)' may also produce zero value.
>
> On 2022/3/10 11:34, Xinlong Lin wrote:
> > On 2022/2/22 11:12, Liang Zhang wrote:
> > > In current async pagefault logic, when a page is ready, KVM relies on
> > > kvm_arch_can_dequeue_async_page_present() to determine whether to deliver
> > > a READY event to the Guest. This function test token value of struct
> > > kvm_vcpu_pv_apf_data, which must be reset to zero by Guest kernel when a
> > > READY event is finished by Guest. If value is zero meaning that a READY
> > > event is done, so the KVM can deliver another.
> > > But the kvm_arch_setup_async_pf() may produce a valid token with zero
> > > value, which is confused with previous mention and may lead the loss of
> > > this READY event.
> > >
> > > This bug may cause task blocked forever in Guest:
> > >    INFO: task stress:7532 blocked for more than 1254 seconds.
> > >          Not tainted 5.10.0 #16
> > >    "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> > >    task:stress          state:D stack:    0 pid: 7532 ppid:  1409
> > >    flags:0x00000080
> > >    Call Trace:
> > >     __schedule+0x1e7/0x650
> > >     schedule+0x46/0xb0
> > >     kvm_async_pf_task_wait_schedule+0xad/0xe0
> > >     ? exit_to_user_mode_prepare+0x60/0x70
> > >     __kvm_handle_async_pf+0x4f/0xb0
> > >     ? asm_exc_page_fault+0x8/0x30
> > >     exc_page_fault+0x6f/0x110
> > >     ? asm_exc_page_fault+0x8/0x30
> > >     asm_exc_page_fault+0x1e/0x30
> > >    RIP: 0033:0x402d00
> > >    RSP: 002b:00007ffd31912500 EFLAGS: 00010206
> > >    RAX: 0000000000071000 RBX: ffffffffffffffff RCX: 00000000021a32b0
> > >    RDX: 000000000007d011 RSI: 000000000007d000 RDI: 00000000021262b0
> > >    RBP: 00000000021262b0 R08: 0000000000000003 R09: 0000000000000086
> > >    R10: 00000000000000eb R11: 00007fefbdf2baa0 R12: 0000000000000000
> > >    R13: 0000000000000002 R14: 000000000007d000 R15: 0000000000001000
> > >
> > > Signed-off-by: Liang Zhang <zhangliang5@xxxxxxxxxx>
> > > ---
> > >    arch/x86/kvm/mmu/mmu.c | 13 ++++++++++++-
> > >    1 file changed, 12 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > > index 593093b52395..8e24f73bf60b 100644
> > > --- a/arch/x86/kvm/mmu/mmu.c
> > > +++ b/arch/x86/kvm/mmu/mmu.c
> > > @@ -3889,12 +3889,23 @@ static void shadow_page_table_clear_flood(struct kvm_vcpu *vcpu, gva_t addr)
> > >        walk_shadow_page_lockless_end(vcpu);
> > >    }
> > >    +static u32 alloc_apf_token(struct kvm_vcpu *vcpu)
> > > +{
> > > +    /* make sure the token value is not 0 */
> > > +    u32 id = vcpu->arch.apf.id;
> > > +
> > > +    if (id << 12 == 0)
> > > +        vcpu->arch.apf.id = 1;
> > > +
> > > +    return (vcpu->arch.apf.id++ << 12) | vcpu->vcpu_id;
> > > +}
> > > +
> > >    static bool kvm_arch_setup_async_pf(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
> > >                        gfn_t gfn)
> > >    {
> > >        struct kvm_arch_async_pf arch;
> > >    -    arch.token = (vcpu->arch.apf.id++ << 12) | vcpu->vcpu_id;
> > This patch is completely OK. But I have a question, can we simplify it to
> > arch.token = (++vcpu->arch.apf.id << 12) | vcpu->vcpu_id;
> > > +    arch.token = alloc_apf_token(vcpu);
> > >        arch.gfn = gfn;
> > >        arch.direct_map = vcpu->arch.mmu->direct_map;
> > >        arch.cr3 = vcpu->arch.mmu->get_guest_pgd(vcpu);
> >
> > .